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Adjacencies

The fact that routers are neighbors is not sufficient to guarantee an exchange of link-state updates; they must form adjacencies to exchange link-state updates. Adjacency is an advanced form of neighborship formed by routers that are willing to exchange routing information after negotiating parameters of such an exchange. Routers reach a FULL state of adjacency when they have synchronized views on a link-state database.

Interface type plays a major role in how the adjacencies are formed. For example, neighbors on point-to-point links always try to become adjacent, while routers attached to broadcast media such as Ethernet can choose to become adjacent only with a subset of neighboring routers on the interface.

Once a router decides to form an adjacency with a neighbor, it starts by exchanging a full copy of its link-state database. The neighbor, in turn, exchanges a full copy of its link-state database with the router. After passing through several neighbor states, the routers become fully adjacent.